1. Field of the Invention
The present invention relates to a steering gear structure of the rack-and-pinion type used in a vehicle.
2. Description of the Prior Art
There is a steering gear of the rack-and-pinion type in which a piston is received within a rack cylinder and reciprocates within the oil-filled rack cylinder so as to move the rack via a pinion associated with a steering wheel according to the rotation of the steering wheel by an operator. A rack-and-pinion steering gear has been proposed in which the piston is elastically attached to one end of the rack in order to partition the rack cylinder into two chambers and to ensure the oil-tightness of the piston which slides smoothly within the rack cylinder by preventing misalignment between the piston and rack.
The above-described conventional steering gear structure of the rack-and-pinion type is exemplified by Japanese Patent Application No. 55-104,929 filed on July 30, 1980 and its unexamined Open No. 56-25058 published on March 10, 1981. The corresponding U.S. patent document is U.S. Pat. No. 4,380,273 to Walter filed on Aug. 1, 1980.
The system construction disclosed in the above-identified document will be described in detail hereinafter with reference to FIGS. 1 and 2 which are disclosed in the above-identified Japanese Application and U.S. Patent documents.
In FIGS. 1 and 2, numeral 1 denotes a pinion shaft rotatably disposed within a gear housing 2 and numeral 3 denotes a rack driven by the pinion shaft 1. In addition, numeral 4 denotes a boss projecting axially from the end face 3b of one end 3a of the rack 3 and having a smaller outer diameter l.sub.2 than the outer diameter l.sub.1 of the rack 3. Numeral 5 denotes a base ring fitted over the boss 4. Numeral 6 denotes a piston mounted on the radial periphery of the base ring 5. An annular projection 7 formed on the inner periphery 6a of the piston 6 is housed within an annular recess 8 formed in the outer periphery 5b of the base ring 5 with a desired clearance in the axial direction. An outer seal ring 11 and an elastic seal ring 12 are received in an outer seal groove 9 formed in the outer periphery 6b of the piston 6. The outer seal ring 11 serves to provide a liquid-tight seal structure between the cylinder 10 and piston 6. The elastic seal ring 12 is arranged inside of the outer seal ring 11. Numeral 13 denotes two elastic seal rings arranged side-by-side between the annular recess 8 of the base ring 5 and annular projection 7 of the piston. Finally, numeral 14 denotes a rivet-shaped bead formation retaining the base ring 5 on the rack 3.
Since in the rack-and-pinion steering gear structure of the construction described above the piston 6 is elastically installed in such a way that there is a clearance in the axial direction with respect to the end 3a of the rack 3 and the piston 6 is urged into elastic contact radially with the wall 10a of the rack cylinder 10 by the elastic seal rings 13, installation errors can be canceled by the play in the piston 6 should the piston 6, rack 3, and cylinder 10 not be accurately mounted coaxially. Consequently, the outer seal ring 11 will always elastically contact the inner periphery 10a of the cylinder 10 without clearance. Hence, the piston 6 partitions the cylinder 10 into the two operation chambers 15, 16 while providing a liquid-tight seal structure therebetween by means of the outer seal ring 11.
In addition, since the piston 6 slides along the inner periphery (or wall) 10a of the cylinder 10 even in the case where a part of or all of the rack 3 is bent or skewed, the piston 6 can smoothly reciprocate within the cylinder 10 without excessive pressure locally on any part of the inner periphery 10a of the cylinder.
However, there are drawbacks in the abovedescribed conventional steering gear structure in that since the base ring 5 must be interposed between the piston 6 and boss 4 and two elastic seal rings 13, 13 must be interposed between the base ring 5 and annular projection 7 of the piston 6, and furthermore the base ring 5 must then be mounted on the rack 3 and thereafter the tip of the boss 4 must be flared to hold it in place, it takes a long time and a high level of workmanship to assemble. In addition, since in order to mount the piston 6 with adequately small axial clearances, the steering gear must include the base ring 5 within which the annular projection 7 is inserted, the profiles and structures of the constituent parts are unavoidably complex and accordingly the cost of machining the parts and so the total cost of assembling the steering gear will be unnecessarily high.